Posted
by
timothyon Thursday July 23, 2009 @05:11PM
from the when-moving-target-is-not-figurative dept.

carstene writes "Qualification rounds for the NASA Centennial Challenge Power beaming contest are underway at the Dryden Flight Research Center. The contest uses a scale model of a space elevator as a race track. Entrants must build a robot to climb a cable, suspended by helicopter, 1 km into the sky without any on board energy storage. The teams are using high power laser beams to transmit power from ground stations to photovoltaic arrays on the robots. If a team can accomplish this at 5 meters per second average speed then they could win up to 2 million dollars. One day this technology could be used to power rovers in shadowed areas of the moon or to recharge electric UAV's in-flight or even a space elevator in the far future. A blog of the event can be found here. Full disclosure: I'm a member of the LaserMotive team that you can follow on twitter, or or via blog."

Last I heard there were bigger problems with space elevators than the energy required to get up there.

A circular geosynchronous orbit in the plane of the Earth's equator has a radius of approximately 42,164 km (from the center of the Earth). A satellite in such an orbit is at an altitude of approximately 35,786 km above mean sea level.

Oh come on. You just know some bozo will figure out how to modulate the power laser so you can enjoy 101 strings doing songs you used to love until you just fucking jump instead of waiting for the rescue...

That's why I like the idea of a Launch Loop better (not that it doesn't have its issues too). It uses kinetic energy to maintain the structure rather than tension so it could in theory be built with modern materials. It also launches in minutes rather than the days, weeks, or even months that some space elevator designs call for. It would have a much higher launch capacity and is built on the ground rather than having to boost a cable into orbit. It doesn't require an anchor weight in high orbit and since the energy for launch is also transfered mechanically you don't have to worry about beaming power anywhere. Finally, it would act as a huge and efficient energy store, meaning we could, in theory, use 100% solar/wind power and use a launch loop as the worlds biggest battery for night time and cloudy days.

So what if the launch loop didn't have the turnarounds at each end? What if the two stations were near one of the poles instead of at the equator? You locate at a latitude just far enough away from the pole to make the circumference of a circular loop equal to 4000km. Then, instead of tossing the cable up in a straight line and have to turn it around at the other end, you toss it in the air, and the earth's rotation carries it around to the other station "halfway around the world", where it's launched up ag

Last I heard there were bigger problems with space elevators than the energy required to get up there.

A circular geosynchronous orbit in the plane of the Earth's equator has a radius of approximately 42,164 km (from the center of the Earth). A satellite in such an orbit is at an altitude of approximately 35,786 km above mean sea level.

Well, you know, in a way, the huge distance by itself isn't that big a problem.

The big problem is the material science, creating macro-scale nanotubes long enough to be woven into a cable or ribbon, and strong enough to support the ribbon itself plus whatever we want to lift. Last I heard (and I'm admittedly not following it closely at all) they could manufacture single nanotubes a meter long, and had nanotubes less than an order of magnitude from the desired tensile strength. But not at the same time. Still, it's promising, but there's a long way to go.

Once you've solved the material science problem, and hopefully made large-scale manufacturing feasible if not cheap, then it's mostly a matter of motivation. Laying down and occasionally carving paths through the mountains for 75,000km of interstate probably sounded daunting, but it got done because there was a perceived need. Between the military uses and commercial uses, I think it would exist for the space elevator too. But it would probably be the DoD who would have the money to do it. With low cost to orbit, Project Thor would be an economical reality. That's my pitch. I'm sure we could add more. Of course we have time, though, because for now, large-scale manufacturing of carbon nanotube cables is still a dream, and thus so is the elevator.:)

You can still do the project in stages, just to a lesser degree. First you make a thin cable that is only useful for small payloads, which will include the next section of cable when its ready, and so on until you have your full-strength cable.

The point wasn't to reduce the tensile strength required; that material science problem still needs to be solved.

The point was that once you have accomplished that and it's a matter of manufacturing and will, you can make use of smaller cables in stages while waiting for the full construction to finish, much like you could use portions of the interstate system before it was done. But instead of making roads that are full width, but not the full length needed, you're making ribbons that are full length but not full width.

Material properties aside, it would be interesting to build a ring around the Earth. The net gravitational force on it would be zero (small variations may have to be accounted for), so it could float above the Earth with zero supports. You could build a series of them, allowing for shorter distances to be travelled at once, which gives you a lot more leeway on power, cable design and weight (mass I guess). Counterbalance the elevator and you won't pull the rings out of the sky.

I recall reading, a few years ago, about a craft intended to lift payloads into orbit which operated by firing lasers at its underside which would ignite a fuel. I guess it's basically this [wikipedia.org].

I'm all for research into all kinds of technologies but to me this almost sounds like a glorified Radio Shack kit; shine a flashlight at a robot to get it to roll around. If it's got photovoltaic cells why even bother with the lasers? Just make the thing solar powered. I suppose this method ensures more power for the rob

One of the founders of LaserMotive, Jordin Kare [wikipedia.org] is the originator of an idea for laser launch [wikipedia.org]. Its a very cool idea that seems very workable for putting small payloads into orbit by heating H gas in a heat exchanger on a rocket with a ground based laser.

As far as why a laser and not solar? The laser is a lot brighter then the sun over the array of the PV array, and the PV array is allot more efficient at the lasers wavelength (color), so you can have a much more compact system. Besides this way you ca

I can see why they'd want to use lasers - how else are you going to focus the energy sufficiently from a distance of 1 or more kilometres?

But why would they use lasers and PV cells when masers could be used instead? Highly directional radio antennas should be both simpler to build and waaaaayyyy more efficient, IMHO, and masers aren't any less efficient than lasers...

Diffraction. Lasers have a wavelength of around 1 micron; the shortest-wavelength microwaves we can make at high power (using gyrotrons, incidentally; unlike lasers, masers are low power, and are now quite obsolete as microwave amplifiers) are around 2 millimeters, 2000 times longer. The antenna/telescope diameter is proportional to wavelength, and the aperture *area* (which is what costs money) is proportional to the wavelength *squared*.

about a craft intended to lift payloads into orbit which operated by firing lasers at its underside

I'm wondering what power could be supplied to the lift-trains (for want of a better description) from orbit. I'd imagine there would be multiple options for powering the cars that are not earth centric.

For example what about lift-trains falling *away* from earth and slowing their velocity beaming that power to trains that are *climbing*. Not all of the power needed to climb but then the climbing (climbers?)

The center of mass not only depends on the positions of the parts, but also on, well, their masses. So if you make your satellite heavy enough, it can be even just one meter above the geosynchronous orbit. Of course there's a tradeoff involved: The heavier the satellite gets, the more expensive it gets to put it there. OTOH, having it at greater distance will also increase the cost. I guess there would be an ideal distance where the cost is minimized.

The lasers are actually infrared and invisible. Ours is 808nm and is very slightly visible to some as a violet glow. For this use lasers are easier to work with then microwaves as they have a much smaller divergence so the transmitter can be much much smaller. For beaming microwaves over these distance you end up with a transmitter that does a fair imitation of a radio telescope.

The lasers are actually infrared and invisible. Ours is 808nm and is very slightly visible to some as a violet glow. For this use lasers are easier to work with then microwaves as they have a much smaller divergence so the transmitter can be much much smaller. For beaming microwaves over these distance you end up with a transmitter that does a fair imitation of a radio telescope.

But at those distances, how can you possibly get the sharks to hold still enough to keep the beam focused?

Are there other reasons besides beam divergence to use a laser to transmit power? Is the atmospheric transmission better (my guess is no) or energy conversion efficiency better for the laser than a microwave?

Beam divergence is the biggest reason I'm aware of. Another sizable reason is kilowatt for kilowatt an infrared laser is a lot cheaper. Diode lasers (especially infrared ones) have collapsed in price in the last few years.

Turns out that divergence is set by the wavelength, larger the wavelength the bigger the minimal divergence. Check out http://en.wikipedia.org/wiki/Diffraction-limited_system . Our lasers wavelength is 808 nanometers. Compare that to say 1 centimeter for microwave and you can see that microwaves will always require a much bigger "lens/mirror" to focus them.

No. In fact, your point is self contradictory. If it means the former, it necessarily includes the latter. It's not possible for the first thing you said it meant to be true while the second is false. Rather like it's impossible for a basket that contains five apples to not contain three apples. Just because you include more than the first thing doesn't mean you no longer include the first thing. If "full disclosure" means including a notice of how you might be biased, including that notice and saying

Exactly, Knowing someone is biased doesn't give you any information into how he might be biased. It's important because instead of reading everything as a skeptic, you could now just be skeptical about his team rocking while the others suxors.

Here is a better example, A judge could potentially be biased because he has met the defendant before. Disclosing this isn't an automatic requirement for a recusal (judicial disqualification) because the disclosure could be that he saw the defendant at a charity dinner

No. In fact, your point is self contradictory. If it means the former, it necessarily includes the latter. It's not possible for the first thing you said it meant to be true while the second is false. Rather like it's impossible for a basket that contains five apples to not contain three apples. Just because you include more than the first thing doesn't mean you no longer include the first thing. If "full disclosure" means including a notice of how you might be biased, including that notice and saying something else as well does not mean you failed to include the notice, so it's still "full disclosure".

First of all, holy crap could you make that any more confusing? And thanks for explaining to me that a basket with 5 apples can't not have 3 apples in it - when you think someone is that wrong, maybe you should consider the possibility that you misunderstood them? People are rarely that dim.

And it doesn't matter anyway, you completely missed my point. I never said what he did wasn't full disclosure, i was just suggesting that that the self promotion was unnecessary.

Kool idea but I would imagine that unless those solar panels could travel down the tether they would be rather expensive to repair and maintain. Being attached to the climber they could easily be maintained when it returns to Earth.

Indeed, it's time that someone invents the carbon nanotube unlimited current room temperature superconductor, so that we can have cables carrying electricity from space without loss, and without collapsing under their own weight.

Solar panels are currently under 20% efficient with the most efficient cells ever produced being around 40%, so let's use that as an estimate. Each square metre of solar panel therefore gives you 400W of power. The amount of energy you need to lift the climber is the potential energy gain: mhg. If we divide both sides of this by time, we get that the power is the mass multiplied by g, multiplied by the vertical speed. We'll use g at the surface, which is around 9.8m/s/s (round it to 10 to make the calcu

This is actually a pretty dangerous job for a helicopter pilot. If his engine fails (which does happen from time to time), he'll be unable to autorotate and will crash fatally. Just like fixed-wing airplanes, helicopters require forward motion to be able to recover from engine failures by gliding to the ground. For this reason, helicopter pilots generally try to avoid hovering unless they're just above the ground; takeoffs and landings are done with forward motion as much as possible.

The scariest thing about this is being on the ground, next to the cable, hooking up a climber with a big Sikorsky S-58 above you. Ever see what happens when a cable under 800lbs of tension snaps? Think Amusement park nightmare.

"Hovering at 20 to 50 feet puts you in the "deadman's curve" - it's a combination or airspeed (0 knots) and altitude (20 to 50 feet) at which a safe autorotation is not possible. So if the engine quits, you're dead."

"Hovering at 20 to 50 feet puts you in the "deadman's curve" - it's a combination or airspeed (0 knots) and altitude (20 to 50 feet) at which a safe autorotation is not possible. So if the engine quits, you're dead."

what brave soul wants to pilot the test helicopter anchoring the top of the beanstalk, while engineers of varying degrees of competence are aiming powerful directed energy beams at an object suspended a short distance below them.

I tend to think of the pilot as pretty brave too, but the contest organizers and NASA have gone to great lengths to make it as safe as possible. For example the helicopter actually hovers at about 1.3km and the lasers all aim in a direction where the helicopter should not be "illuminated". Furthermore while the lasers transmit many kilowatts of power, they are actually fairly spread out, over a square meter or so. They are an eye hazard, but there is no danger of cutting a hole in anything Goldfinger st

Well, sure, the chopper would probably* autorotate down. And it will probably do so in some direction other than straight down. But if it did fall (relatively slowly) straight down, you can add the insult (or injury) of being painted by a high-energy laser ("AAAH, MY EYES! THE GOGGLES DO NOTHING!") to the injury (or severe injury) of a crash-landing.

*There are a few catastrophic failure modes, such as spontaneous rotor system failure, in which autorotation isn't available. OTOH, in those failure modes, lase

My apologies, I was utterly confused there.
I thought you were talking about putting this cannon on the end of a tether, thereby making it orbit to surface. I know realise that you were probably refering to the implications of the lasers, and not the tethering part, and that I am an idiot.

Well, the true reason why aliens are scared is that lasers rays of exactly that type are a side effect of their most effective weapon: The blackhole gun. It works by producing black holes, which then are fired onto the enemy ship. The production process causes laser light to be emitted in some direction; usually that laser is directed into space, exactly to scare the enemy away (actually using that weapon is very expensive, therefore it's a big advantage if the enemy just goes away in fear). Well, actually

Wouldn't a two piston Sterling Engline designed such that it flipped itself over on each cycle be a much better energy down converter than solar cells? Even if your laser is tuned to the solar cell band gap, the amount of energy that you put into the power transfer is a fraction of what you would get as useful energy to the crawler. With a Sterling Engine you could just use mirrors to focus sunlight to power the device.

I've seen some pretty flimsy Stirling Engine implementations. Some even have foam pistons, but you are probably still right about the P/W ratio. I've never seen the flimsy ones do much more than turn. Attaining 5m/sec = 300m/min with an engine capable of turning at 1200 rpm would mean that the engine would have to be capable of lifting itself.25m =~ 8 inches per turn. This does not seem like an impossible number, but it doesn't sound easy either. It would probably be necessary to put it into motion at

A diode laser, like we are using is also about 50% efficient in turning electrons into photons, so under ideal conditions you get around 25% of the electricity out the other end that you put in. I will point out that you still have to heat the Stirling engine with something like a laser so it has exactly the same lose on that end as beaming to a PV does.

No! Don't do that! The laser would be applying a pressure in the wrong direction. If you must, reflect it on a mirror on earth so the light can bounce back up to the craft. (In all actuality, the force from the light pressure is probably insignificant.)

I'll supply the power over a single conductive cable 1 km long if you'll supply the robot to climb it. We can share the prize. I'm ready to demonstrate. To see how I do it see http://www.corridor.biz/FullArticle.pdf [corridor.biz]
n6gn